Ferroelectric thin-film laminated substrate, ferroelectric thin-film device,and manufacturing method of ferroelectric thin-film laminated substrate

ABSTRACT

There is provided a ferroelectric thin-film laminated substrate, including a substrate, and further including a lower electrode layer, a ferroelectric thin-film layer, an upper electrode adhesive layer, and an upper electrode layer being sequentially stacked on the substrate, in which: the lower electrode layer is made of platinum or a platinum alloy; the ferroelectric thin-film layer is made of a sodium potassium niobate (typical chemical formula of (K 1-x Na x )NbO 3 , 0.4≤x≤0.7); the upper electrode layer is made of gold; the upper electrode adhesive layer is made of a metal that has less oxidizability than titanium and can make a solid solution alloy without generating an intermetallic compound with gold; and a part of the upper electrode adhesive layer and a part of the upper electrode layer are alloyed.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a ferroelectric thin-film device, and more particularly to a ferroelectric thin-film laminated substrate having lead-free niobate-based ferroelectrics, a ferroelectric thin-film device cut out of the thin-film laminated substrate, and a method for manufacturing the thin-film laminated substrate.

DESCRIPTION OF BACKGROUND ART

Ferroelectrics are very attractive substances because of their peculiar characteristics (such as very high relative permittivity, and good pyroelectric, piezoelectric and ferroelectric properties). So, various devices (such as ceramic multilayer capacitors, pyroelectric devices, piezoelectric devices and ferroelectric memories) have been developed and put into use utilizing such peculiar properties. Typical ferroelectrics are perovskite materials such as barium titanate (BaTiO₃) and lead zirconate titanate (Pb(Zr_(1-x)Ti_(x))O₃, PZT). Of these, lead zirconate titanates (PZTs) provide relatively excellent polarization and piezoelectric properties and are therefore most widely used.

Lead-containing PZTs are specified hazardous substances. However, because there are currently no suitable commercially available alternative pyroelectric or piezoelectric materials, PZTs are exempt from the RoHS directive (the directive on the restriction of the use of specified hazardous substances in electrical and electronic equipment enforced by the European Union and Council of Europe). However, with the growing worldwide responsibility towards global environment conservation, a strong demand exists for development of pyroelectric and piezoelectric devices using lead-free ferroelectric materials.

Also, with the recent trend toward smaller and lighter electronic devices, there is an increasing need for ferroelectric thin-film devices in which a thin-film technology is utilized.

Herein, pyroelectric and piezoelectric thin-film devices will be described below as representatives of such ferroelectric thin-film devices. Piezoelectric devices utilize the piezoelectric effect of a ferroelectric material, and are widely used as functional devices such as actuators and stress sensors. Actuators generate a displacement or vibration in response to an applied voltage to a ferroelectric (piezoelectric) material. Stress sensors generate a voltage in response to a strain produced in a piezoelectric material. Pyroelectric devices detect light (including infrared light) utilizing the pyroelectric effect of a ferroelectric material, and are widely used as infrared human body sensors, etc.

Examples of piezoelectric devices utilizing a lead-free piezoelectric material are described below. Patent Literature 1 discloses a piezoelectric thin-film device including, on a substrate, a lower electrode, a piezoelectric thin-film and an upper electrode. The piezoelectric thin-film is made of an alkali niobate-based perovskite dielectric material of a chemical formula (Na_(x)K_(y)Li_(z))NbO₃ (where 0<x<1, 0<y<1, 0≤z<1, and x+y+z=1). A buffer layer of a perovskite crystal structure material is formed between the piezoelectric thin-film and the lower electrode. The perovskite buffer layer is highly preferentially (001), (100), (010) or (111) oriented. According to this Patent Literature 1, the piezoelectric thin-film device utilizing the lead-free lithium potassium sodium niobate thin-film exhibits sufficient piezoelectric properties.

The practical use of the lead-free piezoelectric thin-film device requires reliability (durability) in long-term use in addition to improvement in piezoelectric properties. Various techniques intended for that have been developed.

For example, Patent Literature 2 discloses a piezoelectric film element including: a substrate; a lower electrode structure provided on the substrate; a KNN piezoelectric film provided on the lower electrode structure; at least one diffusion prevention structure provided on the KNN piezoelectric film; and an upper electrode structure provided on the diffusion prevention structure. The upper electrode structure includes an upper electrode made of an Au layer. The diffusion prevention structure includes, in order from the KNN piezoelectric film, an intermediate adhesive layer and an Au diffusion prevention layer that is provided on the intermediate adhesive layer and prevents diffusion of Au from the upper electrode to the KNN piezoelectric film.

Patent Literature 2 also discloses that the Au diffusion prevention layer is made of a Pt layer or a Pt alloy layer and that the intermediate adhesive layer is made of a Cr layer, a Ta layer, a Ti layer, a Ti oxide layer, or a Ti nitride layer. According to Patent Literature 2, a practical piezoelectric film device in which the electric insulation of the KNN piezoelectric film hardly deteriorates in long-term use and a piezoelectric film apparatus including the piezoelectric film device can be provided.

CITATION LIST Patent Literature

Patent Literature 1: JP 2007-019302 A

Patent Literature 2: JP 2014-060267 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As mentioned above, niobate-system ferroelectrics (such as a potassium sodium niobate, typical chemical formula of (K_(1-x)Nax)NbO₃) are among particularly promising materials as lead-free ferroelectrics. However, because niobate-based ferroelectric materials are relatively new materials, lamination structures of thin-film devices and manufacturing processes thereof are still under development. In order to put such thin-film devices using niobate-based ferroelectric materials as alternatives for PZT thin-film devices to practical use and mass production, it is very important to develop and establish the lamination structures and manufacturing processes of the thin-film devices satisfying various requirements for the practical use and mass production.

For example, it is often the case that wire bonding is used to make electric connection in incorporating an electronic component into electronic equipment, and an indicator of the bondability of wire bonding is bond strength (tensile strength) between a bonding wire and an electrode pad. The piezoelectric thin-film device of Patent Literature 2 uses an upper electrode made of a gold layer that is suitable for wire bonding from a standpoint of bondability with respect to a bonding wire (e.g., a gold bonding wire). However, from a mass productivity standpoint, a bond strength higher than before (e.g., a tensile strength of 60 mN or more) is recently required. Thus, it has been difficult for the technique of Patent Literature 2 to meet such need.

In view of the foregoing, it is an objective of the present invention to provide a niobate-based ferroelectric thin-film laminated substrate that enables a bond strength higher than before when wire bonding is performed on a thin-film device cut out of the thin-film laminated substrate, a ferroelectric thin-film device cut out of the thin-film laminated substrate, and a method for manufacturing the thin-film laminated substrate.

Solution to Problems

(I) According to one aspect of the present invention, there is provided a ferroelectric thin-film laminated substrate, including a substrate, and further including a lower electrode layer, a ferroelectric thin-film layer, an upper electrode adhesive layer, and an upper electrode layer, which are sequentially stacked on the substrate. In the ferroelectric thin-film laminated substrate, the lower electrode layer is made of platinum (Pt) or a Pt alloy; the ferroelectric thin-film layer is made of a sodium potassium niobate (typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7); the upper electrode layer is made of gold (Au); the upper electrode adhesive layer is made of a metal which has less oxidizability than titanium and can make a solid solution alloy without generating an intermetallic compound with gold; and a part of the upper electrode adhesive layer and a part of the upper electrode layer are alloyed.

In the above aspect (I) of the invention, the following modifications and changes can be made.

(i) The upper electrode adhesive layer may be made of one selected from nickel (Ni), cobalt (Co), tungsten (W), and molybdenum (Mo).

(ii) The upper electrode adhesive layer may have a thickness of 5 nm or more but 50 nm or less.

(iii) A lower electrode adhesive layer may be further stacked between the substrate and the lower electrode layer, the lower electrode adhesive layer being made of titanium (Ti) and/or a Ti oxide.

(iv) The lower electrode layer may have a main surface with a (111) crystal plane preferential orientation; and the ferroelectric thin-film layer may have a crystal system of a pseudo cubic system or a tetragonal system, and a main surface with a (011) crystal plane preferential orientation.

(v) The substrate may be a silicon substrate having a thermally oxidized film on its surface.

(II) According to another aspect of the invention, there is provided a ferroelectric thin-film device characterized by using the ferroelectric thin-film laminated substrate of the invention described above.

(III) According to still another aspect of the invention, there is provided a manufacturing method of the ferroelectric thin-film laminated substrate of the invention described above. The manufacturing method includes: a lower electrode layer formation step of forming the lower electrode layer on the substrate; a ferroelectric thin-film layer formation step of forming the ferroelectric thin-film layer on the lower electrode layer; an upper electrode adhesive layer formation step of forming the upper electrode adhesive layer on the ferroelectric thin-film layer; and an upper electrode layer formation step of forming the upper electrode layer on the upper electrode adhesive layer. The upper electrode layer formation step includes a process of depositing the upper electrode layer under a temperature environment of 50° C. or more but 200° C. or less to alloy a part of the upper electrode adhesive layer with a part of the upper electrode layer.

Advantages of the Invention

According to the present invention, it is possible to provide a niobate-based ferroelectric thin-film laminated substrate that enables a bond strength higher than before when wire bonding is performed on a thin-film device cut out of the thin-film laminated substrate, a ferroelectric thin-film device cut out of the thin-film laminated substrate, and a method for manufacturing the thin-film laminated substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a cross-sectional view of a ferroelectric thin-film laminated substrate according to the present invention; and

FIG. 2 is a graph showing exemplary relationships between polarization value and applied voltage with respect to KNN thin-film devices of Comparative Example 2 and Examples 1 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have focused on a sodium potassium niobate (typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7; hereinafter referred to as KNN) as a lead-free ferroelectric material that can be expected to exhibit a ferroelectricity comparable to that of a PZT (typical chemical formula: Pb(Zr_(1-x)Ti_(x))O₃), and have conducted research and development with an aim to put KNN thin-film devices into practical use. In the research and development, the inventors carried out intensive investigations on a laminated structure of the KNN thin-film device (particularly, the laminated structure of an upper electrode) that enables a bond strength higher than before when wire bonding is performed on the KNN thin-film device, and on a process of manufacturing the laminated structure of KNN thin-film device.

As a result, it is found that the above can be achieved by using, as an upper electrode intermediate layer formed between the ferroelectric thin-film layer and the upper electrode layer, a metal that has less oxidizability than Ti (means a metal that hardly oxidizes, indicating a metal having a larger ionization potential) and can make a solid solution alloy without generating an intermetallic compound with Au. The present invention has been made on the basis of this new finding.

Preferred embodiments of the invention will be described below with reference to the accompanying drawings. However, the invention is not limited to the specific embodiments described below, but various combinations and modifications are possible without departing from the spirit and scope of the invention.

FIG. 1 is a schematic illustration showing a cross-sectional view of a ferroelectric thin-film laminated substrate according to the invention. As illustrated in FIG. 1, a ferroelectric thin-film laminated substrate 10 of the invention has a structure in which on a substrate 1, a lower electrode adhesive layer 2, a lower electrode layer 3, a ferroelectric thin-film layer 4, an upper electrode adhesive layer 5, and an upper electrode layer 6 are stacked in this order. The ferroelectric thin-film device according to the invention can be obtained as a chip with a desired shape by cutting out of the ferroelectric thin-film laminated substrate 10 of the invention. Meanwhile, the upper electrode adhesive layer 5 and the upper electrode layer 6 may be formed at the stage of the ferroelectric thin-film laminated substrate 10 or may be formed after the device is cut out into a desired shaped chip.

A detailed description is given in accordance with a procedure of manufacturing the ferroelectric thin-film laminated substrate 10.

First, a substrate 1 is prepared. A material of the substrate 1 is not particularly limited, and may be properly selected based on applications of the ferroelectric thin-film devices. For example, silicon (Si), SOI (Silicon on Insulator), quartz glass, gallium arsenide (GaAs), gallium nitride (GaN), sapphire (Al₂O₃), magnesium oxide (MgO), zinc oxide (ZnO), and strontium titanate (SrTiO₃) may be used. From the view point of cost, using Si substrates is preferable among these materials. When an electrically conductive material is used as the substrate 1, its surface is preferably covered with an electrical insulating film (e.g. an oxide film). There is no particular limitation on a method of forming the oxide film. For example, thermal oxidation and chemical vapor deposition (CVD) are suitable.

Next, the lower electrode adhesive layer 2 is formed on the substrate 1. Although the formation of the lower electrode adhesive layer 2 is not essential, the lower electrode adhesive layer 2 is preferably formed in order to increase adhesiveness between the substrate 1 and the lower electrode layer 3. Furthermore, it is preferable in terms of adhesiveness or environmental resistance that the material of the lower electrode adhesive layer 2 is, e.g., Ti and/or a Ti oxide having a thickness of 1 to 3 nm. Ti is an easy oxidability metal (meaning a metal that oxidizes easily; a metal having a small ionization potential, e.g., first ionization potential), which tends to bind to an oxygen atom present on a surface of the substrate 1 into the form of a titanium oxide (atomic bond state), so that high adhesiveness can be secured. There is no particular limitation on a method of forming the lower electrode adhesive layer 2. For example, the lower electrode adhesive layer 2 is formed preferably by a physical vapor deposition method such as sputtering, thermal evaporation, and electron beam evaporation.

Next, a lower electrode layer 3 is formed on the lower electrode adhesive layer 2. As a material of the lower electrode layer 3, Pt or a Pt alloy (alloy containing mainly platinum) is preferable. The lower electrode layer 3 is an underlayer of a ferroelectric thin-film layer 4, and then in order to improve crystal orientation of the ferroelectric thin-film layer 4, the lower electrode layer 3 preferably has a main surface with one of a (001) crystal plane, a (110) crystal plane, and a (111) crystal plane preferential orientation. Furthermore, the lower electrode layer 3 preferably has a crystal texture consisting of columnar crystal grains. There is no particular limitation on a method for forming the lower electrode layer 3. For example, the lower electrode layer 3 is formed preferably by a physical vapor deposition method such as sputtering, thermal evaporation, and electron beam evaporation.

Next, the ferroelectric thin-film layer 4 is formed on the lower electrode layer 3. In the present invention, a lead-free ferroelectric of a sodium potassium niobate (typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7; referred to as KNN) is preferable as a material of the ferroelectric thin-film layer 4. In order to achieve sufficient ferroelectric performance, a crystal system of the ferroelectric thin-film layer 4 is preferably a pseudo cubic system or a tetragonal system; and the ferroelectric thin-film layer 4 preferably has a main surface with a (001) crystal plane preferential orientation.

There is no particular limitation on a method for forming the ferroelectric thin-film layer 4 (KNN thin-film) as long as a desired KNN thin-film layer is obtained. Preferable formation methods include sputtering using a sintered body target having a desired chemical composition, electron beam evaporation, and pulsed laser deposition. These formation methods have advantages of excellent chemical composition controllability and crystal orientation controllability of the KNN crystal.

The KNN thin-film layer 4 may contain one or more selected from lithium (Li), tantalum (Ta), antimony (Sb), calcium (Ca), copper (Cu), barium (Ba) and Ti in a total amount of 5 atomic percent or less.

Next, an upper electrode adhesive layer 5 is formed on the ferroelectric thin-film layer 4. It is important that the upper electrode adhesive layer 5 plays a role of increasing adhesiveness between the ferroelectric thin-film layer 4 and the upper electrode layer 6 (role of an adhesive layer), and does not have adverse influences on the performance and property (e.g., ferroelectricity) of the thin-film device. The present invention is best characterized by a combination of the upper electrode adhesive layer 5 and the upper electrode layer 6.

The material of the upper electrode adhesive layer 5 is preferably a metal that has less oxidizability (meaning a metal that hardly oxidizes; a metal having a larger ionization potential, e.g., first ionization potential) than Ti and can make a solid solution alloy without generating an intermetallic compound with Au. For example, one selected from Ni, Co, W and Mo is preferably used. These metals are more resistant to oxidation than Ti. Therefore, the percentage (probability) of generating an oxide during deposition is sufficiently small. For example, it is considered that an atomic bond state with respect to oxygen is generated only in an interface region between the ferroelectric thin-film layer 4 and the upper electrode adhesive layer 5.

In addition, the aforementioned metals (Ni, Co, W and Mo) can be a solid solution alloy without generating an intermetallic compound with Au. Therefore, it is considered that during deposition of an upper electrode layer in a subsequent process, the metals are alloyed with a part of Au of the upper electrode layer 6 to generate a solid solution alloy. Because an intermetallic compound is not generated, the origin of breakage or fracture is hardly generated, and the solid solution enables mixing at atomic level. As a result, it is considered that high integrity and adhesiveness can be obtained, so that a high bond strength can be obtained.

The upper electrode adhesive layer 5 preferably has a thickness (average film thickness) of 5 nm or more but 50 nm or less. When the thickness of the upper electrode adhesive layer 5 is less than 5 nm, an alloying reaction with Au of the upper electrode layer 6 is insufficient (shortage of the amount of solid solution alloy to be generated), resulting in an insufficient effect of improving bond strength. When the thickness of the upper electrode adhesive layer 5 exceeds 50 nm, the effect of improving bond strength is saturated, and the processing cost of micro fabrication (e.g., dry etching) of the upper electrode adhesive layer 5 and the upper electrode layer 6 increases.

Next, the upper electrode layer 6 is formed on the upper electrode adhesive layer 5. According to the invention, Au is preferably used as a material of the upper electrode layer 6 from a standpoint of the bond strength of wire bonding (bondability to the bonding wire). There is also no particular limitation on a method of forming the upper electrode layer 6. Similarly to the case of the lower electrode layer 3, a physical vapor deposition method (e.g., sputtering, thermal evaporation, and electron beam evaporation) may be used preferably.

The deposition of the upper electrode layer 6 is preferably carried out under a temperature condition at 50° C. or more but 200° C. or less, more preferably at 100° C. or more but 200° C. or less. Depositing the upper electrode layer 6 at a predetermined temperature enables acceleration of an alloying reaction (generation of a solid solution alloy) between a part of the upper electrode adhesive layer 5 and a part of the upper electrode layer 6. When the temperature during deposition is less than 50° C., the alloying reaction is insufficient. When the temperature during deposition exceeds 200° C., the metal of the upper electrode adhesive layer 5 is prone to be oxidized, preventing the alloying reaction. In addition, excessive oxidization of the upper electrode adhesive layer 5 is highly prone to lead to deficiency of oxygen in the ferroelectric thin-film layer 4, which can be a cause of degradation of the ferroelectricity.

EXAMPLES

The present invention will be described more specifically below by way of examples. However, the invention is not limited to the specific examples below.

[Preparation of Ferroelectric Thin-Film Laminated Substrate]

On the basis of the thin-film laminated structure (substrate 1, lower electrode adhesive layer 2, lower electrode layer 3, ferroelectric thin-film layer 4, upper electrode adhesive layer 5, upper electrode layer 6) illustrated in FIG. 1, several kinds of ferroelectric thin-film laminated substrates having a different combination of the upper electrode adhesive layer 5 and the upper electrode layer 6 were produced. As the substrate 1, an Si substrate with a thermal oxide film (four-inch wafer having a (100) plane orientation, a wafer thickness of 0.525 mm, a thermal oxide film thickness of 200 nm, a substrate surface roughness Ra=0.86 nm) was used.

In the film formation steps below, the thickness of each layer (lower electrode adhesive layer 2, lower electrode layer 3, ferroelectric thin-film layer 4, upper electrode adhesive layer 5, upper electrode layer 6) was controlled by controlling the film formation time based on the film formation rate determined in advance. Also, the thickness measurement for calculation of each film formation rate was conducted by the X-ray reflectivity technique using an X-ray diffractometer (X'Pert PRO MRD, available from PANalytical B.V., Spectris Co., Ltd.).

First, a Ti layer having a thickness of 2.2 nm was deposited on the Si substrate 1 by RF magnetron sputtering as the lower electrode adhesive layer 2 to enhance adhesion between the substrate 1 and the lower electrode layer 3. Subsequently, a Pt layer having a thickness of 205 nm was deposited by RF magnetron sputtering as the lower electrode layer 3 on the Ti lower electrode adhesive layer 2. The sputtering conditions for the lower electrode adhesive layer 2 and the lower electrode layer 3 were as follows: targets of pure Ti (for the Ti adhesive layer) and pure Pt (for the Pt electrode layer); discharge power of 200 W; sputtering gas of Ar; and gas pressure of 2.5 Pa. The sputtering was carried out using a RF sputtering machine (SH-350-T10, available from ULVAC, Inc.) (the same machine was used in all the remaining sputtering processes described below). The lower electrode layer 3 was subjected to the surface roughness measurement and observed to have an arithmetic mean roughness Ra of 0.86 nm or less.

After the formation of the Pt lower electrode layer 3, a KNN ((K_(0.35)Na_(0.65))NbO₃) thin-film having a thickness of 1.9 μm was deposited by RF magnetron sputtering as the ferroelectric thin-film layer 4 on the lower electrode layer 3. The sputtering condition for the KNN thin-film 4 was as follows: target of sintered (K_(0.35)Na_(0.65))NbO₃; substrate temperature of 400 to 600° C.; discharge power of 700 to 800 W; sputtering gas of O₂/Ar mixture (O₂/Ar ratio=0.005); and gas pressure of 0.3 to 1.3 Pa.

(Evaluation of Crystal Systems of KNN Thin-Film)

Normally, perovskite KNN crystals have a tetragonal structure in which the c-axis is longer than the a-axis (c/a>1). That is, when formed KNN crystals satisfy the crystalline condition of “c/a>1”, the crystals are more stable and the crystallinity thereof is high. When an electric field is applied along the c-axis of a perovskite-type ferroelectric crystal with a small initial strain, a larger polarization (and thus a higher gain in piezoelectric or ferroelectric performance) is obtained.

On the other hand, unlike bulk crystals, thin-film crystals formed on a substrate are prone to distortion caused by the influence of the substrate or the underlayer. Specifically, the KNN thin-film formed on a substrate may have a pseudo cubic crystal system with “c/a≤1” (herein meaning “a crystal system closer to a cubic system than to a proper tetragonal system”) or a tetragonal crystal system with “c/a>1” (herein meaning “a crystal system closer to a proper tetragonal system”). Therefore, the crystal system of each of the KNN thin-films formed above was evaluated by X-ray diffraction (XRD). The results showed that the KNN thin-film laminated substrates, each of which was provided with the KNN thin-film mainly having a tetragonal system with “c/a>1”, were obtained.

Preparation of Comparative Example 1

A Ti layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 on the KNN thin-film layer 4 by electron beam evaporation. Subsequently, an Au layer having a thickness of 300 nm was deposited as the upper electrode layer 6 on the Ti upper electrode adhesive layer 5 by electron beam evaporation. The depositions of the upper electrode adhesive layer 5 and the upper electrode layer 6 (by the electron beam evaporation) of Comparative Example 1 were carried out without substrate heating. Thus, a KNN thin-film laminated substrate of Comparative Example 1 was produced. The electron beam evaporation was carried out using an electron beam evaporation machine (EX-400-008, available from ULVAC, Inc.) (the same machine was used in all the remaining electron beam evaporation processes described below).

Preparation of Comparative Example 2

Another KNN thin-film laminated substrate as Comparative Example 2 was produced in the same manner as Comparative Example 1 described above, except that the depositions (electron beam evaporation) of the upper electrode adhesive layer 5 and the upper electrode layer 6 were carried out under the condition where a substrate temperature was 100° C.

Preparation of Comparative Example 3

Another KNN thin-film laminated substrate as Comparative Example 3 was produced in the same manner as Comparative Example 2 described above, except that a Cr layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 on the KNN thin-film layer 4 by electron beam evaporation (substrate temperature of 100° C.)

Preparation of Comparative Example 4

Another KNN thin-film laminated substrate as Comparative Example 4 was produced in the same manner as Comparative Example 2 described above, except that an Ni layer having a thickness of 2 nm was deposited as the upper electrode adhesive layer 5 on the KNN thin-film layer 4 by electron beam evaporation (substrate temperature of 100° C.).

Preparation of Example 1

Another KNN thin-film laminated substrate as Example 1 was produced in the same manner as Comparative Example 2 described above, except that an Ni layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 on the KNN thin-film layer 4 by electron beam evaporation (substrate temperature of 100° C.).

Preparation of Example 2

Another KNN thin-film laminated substrate as Example 2 was produced in the same manner as Comparative Example 2 described above, except that an Ni layer having a thickness of 50 nm was deposited as the upper electrode adhesive layer 5 on the KNN thin-film layer 4 by electron beam evaporation (substrate temperature of 100° C.).

Preparation of Examples 3 to 5

Another KNN thin-film laminated substrate as Example 3 was produced in the same manner as Comparative Example 2 described above, except that a Co layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 by electron beam evaporation (substrate temperature of 100° C.). Another KNN thin-film laminated substrate as Example 4 was produced in the same manner as Comparative Example 2 described above, except that a W layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 by electron beam evaporation (substrate temperature of 100° C.). In addition, another KNN thin-film laminated substrate as Example 5 was produced in the same manner as Comparative Example 2 described above, except that an Mo layer having a thickness of 10 nm was deposited as the upper electrode adhesive layer 5 by electron beam evaporation (substrate temperature of 100° C.)

[Preparation of Ferroelectric Thin-Film Device]

Small pieces (20 mm×20 mm) were cut out of the KNN thin-film laminated substrates of the aforementioned Comparative Examples 1 to 4 and Examples 1 to 5 to produce measurement/evaluation KNN thin-film devices of Comparative Examples 1 to 4 and Examples 1 to 5.

[Evaluation of Characteristics of Ferroelectric Thin-Film Device]

(1) Evaluation of Wire Bonding Bond Strength

Wire bonding using an Au bonding wire (diameter of 25 μm) was carried out on the resultant KNN thin-film device, and then the bond strength (tensile strength) between the upper electrode layer and the Au bonding wire was measured. A tensile strength of 60 mN or more was determined to be “passed” and a tensile strength of less than 60 mN was determined to be “failed”. Results are shown in Table 1.

TABLE 1 Results of measurement of tensile strength between upper electrode layer and Au bonding wire. Upper Substrate electrode temper- adhesive ature layer during Type Thick- Upper deposi- Tensile of ness electrode tion strength Passed/ metal (nm) layer (° C.) (mN) Failed Comparative Ti 10 Au No 20 Failed Example 1 heating Comparative 100 55 Failed Example 2 Comparative Cr 10 Au 100 50 Failed Example 3 Comparative Ni 2 Au 100 55 Failed Example 4 Example 1 10 95 Passed Example 2 50 97 Passed Example 3 Co 10 Au 100 70 Passed Example 4 W 10 Au 100 75 Passed Example 5 Mo 10 Au 100 70 Passed

As shown in Table 1, for Comparative Example 1, the tensile strength is insufficient and then is failed. This is because the substrate was not heated during depositions of the upper electrode adhesive layer 5 and the upper electrode layer 6, and thus the bondability between the KNN thin-film layer 4 and the upper electrode adhesive layer 5 and the bondability between the upper electrode adhesive layer 5 and the upper electrode layer 6 would be insufficient.

In the case of Comparative Example 2 where the substrate was heated to 100° C. during depositions of the upper electrode adhesive layer and the upper electrode layer, the tensile strength is increased as compared to that of Comparative Example 1, but does not meet the acceptance criterion (60 mN). This is because Ti of the upper electrode adhesive layer 5 would be alloyed with Au of the upper electrode layer 6 during deposition to generate an intermetallic compound.

According to Comparative Example 3 in which the type of metal of the upper electrode adhesive layer 5 is Cr, the tensile strength does not meet the acceptance criterion (60 mN) and is failed. This is because Cr is a metal with equivalent or higher easy-oxidability characteristics than Ti and thus a Cr oxide would be generated excessively.

According to Comparative Example 4 in which the type of metal of the upper electrode adhesive layer 5 is Ni and the thickness thereof is outside of the requirement of the invention, the tensile strength does not meet the acceptance criterion (60 mN) and is failed. This is because the Ni layer is too thin, resulting in that an alloying reaction with Au of the upper electrode layer 6 would be insufficient.

By contrast, it is confirmed that all of Examples 1 to 5 of the invention satisfy the acceptance criterion (60 mN). It should be noted that according to a comparison between Examples 1 and 2, it is observed that the influence of the thickness of the upper electrode adhesive layer 5 is reduced when the wire bonding bond strength meets the acceptance criterion.

[Evaluation of Ferroelectric Characteristics]

The obtained KNN thin-film devices were measured for the polarization using a ferroelectric characteristics analyzer. FIG. 2 is a graph showing exemplary relationships between polarization value and applied voltage with respect to the KNN thin-film devices of Comparative Example 2 and Examples 1 and 4.

As shown in FIG. 2, it is found that the polarization property (hysteresis loop of polarization value) of Comparative Example 2 (upper electrode layer/upper electrode adhesive layer=Au/Ti) is more deteriorated than that of Example 1 (upper electrode layer/upper electrode adhesive layer=Au/Ni) and that of Example 4 (upper electrode layer/upper electrode adhesive layer=Au/W). This is because a Ti oxide would be excessively generated since Ti is an easy oxidability metal and because oxygen deficiency would be occurred in the KNN thin-film layer. In contrast, it is confirmed that the polarization properties of Examples 1 and 4 according to the invention are almost identical and are substantially unchanged.

Regarding the thickness of the upper electrode adhesive layer 5, it was separately confirmed that equivalent results were obtained from Examples 1 and 2. In addition, regarding the type of metal forming the upper electrode adhesive layer 5, it was separately confirmed that results equivalent to those of Example 1 were obtained from Examples 3 to 5.

The above embodiments and examples of the invention as well as the appended claims and figures show multiple characterizing features of the invention in specific combinations. The skilled person will easily be able to consider further combinations or sub-combinations of these features in order to adapt the invention as defined in the claims to his specific needs.

LEGEND

-   -   1 . . . substrate;     -   2 . . . lower electrode adhesive layer;     -   3 . . . lower electrode layer;     -   4 . . . ferroelectric thin-film layer;     -   5 . . . upper electrode adhesive layer;     -   6 . . . upper electrode layer; and     -   10 . . . ferroelectric thin-film laminated substrate. 

The invention claimed is:
 1. A ferroelectric thin-film laminated substrate, comprising a substrate, and further comprising a lower electrode layer, a ferroelectric thin-film layer, an upper electrode adhesive layer, and an upper electrode layer being sequentially stacked on the substrate, wherein: the lower electrode layer is made of platinum or a platinum alloy; the ferroelectric thin-film layer is made of a sodium potassium niobate (typical chemical formula of (K_(1-x)Na_(x))NbO₃, 0.4≤x≤0.7); the upper electrode layer is made of gold; the upper electrode adhesive layer is made of a metal that has less oxidizability than titanium and can make a solid solution alloy without generating an intermetallic compound with gold; and a part of the upper electrode adhesive layer and a part of the upper electrode layer are alloyed.
 2. The ferroelectric thin-film laminated substrate according to claim 1, wherein the upper electrode adhesive layer is made of one selected from nickel, cobalt, tungsten, and molybdenum.
 3. The ferroelectric thin-film laminated substrate according to claim 1, wherein the upper electrode adhesive layer has a thickness of 5 nm or more but 50 nm or less.
 4. The ferroelectric thin-film laminated substrate according to claim 1, wherein: a lower electrode adhesive layer is further stacked between the substrate and the lower electrode layer; and the lower electrode adhesive layer is made of titanium and/or a titanium oxide.
 5. The ferroelectric thin-film laminated substrate according to claim 1, wherein: the lower electrode layer has a main surface with a (111) crystal plane preferential orientation; and the ferroelectric thin-film layer has a crystal system of a pseudo cubic system or a tetragonal system, and has a main surface with a (011) crystal plane preferential orientation.
 6. The ferroelectric thin-film laminated substrate according to claim 1, wherein the substrate is a silicon substrate having a thermally oxidized film on its surface.
 7. A ferroelectric thin-film device using the ferroelectric thin-film laminated substrate according to claim
 1. 8. A manufacturing method of the ferroelectric thin-film laminated substrate according to claim 1, the manufacturing method comprising: a lower electrode layer formation step of forming the lower electrode layer on the substrate; a ferroelectric thin-film layer formation step of forming the ferroelectric thin-film layer on the lower electrode layer; an upper electrode adhesive layer formation step of forming the upper electrode adhesive layer on the ferroelectric thin-film layer; and an upper electrode layer formation step of forming the upper electrode layer on the upper electrode adhesive layer, wherein the upper electrode layer formation step includes a process of depositing the upper electrode layer under a temperature environment of 50° C. or more but 200° C. or less to alloy a part of the upper electrode adhesive layer with a part of the upper electrode layer. 